1. Technical Field
The present invention relates to a liquid crystal composition, a liquid crystal element, a reflective display material, and a light-controlling material, and in particular, to a liquid crystal composition, a liquid crystal element, a reflective display material, and a light-controlling material that are favorably used in guest-host systems.
2. Related Art
Properties demanded for display include high visibility and low power consumption. Display devices in a guest-host system have been known as liquid crystal elements (liquid crystal display elements) satisfying these requirements and are highly anticipated as display devices that allow bright display and are suitable for reflective display. For example, liquid crystal compositions containing a dichroic dye having a particular substituent and a host liquid crystal and display devices in the guest-host system have been disclosed (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2004-75,821).
However, nematic liquid crystals, which have no memory property, require application of voltage for continuous display. In contrast, a method of using a nonvolatile smectic A liquid crystal having a memory property has been proposed, but this liquid crystal also has a problem in that a large amount of energy, such as heat, is needed for changing a displayed image.
Alternatively, “dual-frequency-addressing methods” are known, in which the orientation of liquid crystal is reversibly changed by an applied electric field by using a dual-frequency-addressable liquid crystal that has a dielectric anisotropy Δ∈ changing from positive to negative when the frequency of the applied voltage is increased (see, for example, Applied Physics Letters, Vol. 25, No. 4, pp. 186 to 188 (1974) and Applied Physics Letters, Vol. 41, No. 8, pp. 697 to 699 (1982)). Some smectic A liquid crystals which enable switching of the orientation of liquid crystal actively and exhibit dual frequency addressing have been reported (see, for example, Mol. Cryst. Liq. Cryst., 49, pp. 83-87 (1978)). However, such a system still has problems of very large voltage being required and slow response time. Accordingly, there exists a demand for a nonvolatile liquid crystal element that responds at low voltage in a short period of time.
Along with increased concern about the environment, materials allowing regulation of light quantity electrically, so-called electrical light-controlling materials, are increasing in importance. For example, electrochromic methods of using an oxidation-reduction reaction and polymer-dispersion liquid crystal (PDLC) methods of using a composite of a liquid crystal and a polymer have been proposed as such electrical light-controlling materials. However, the electrochromic systems have problems concerning the difficulty in increasing the area these of due to addressing with electrical current and remaining issues regarding the durability of the electrochromic colorant, while the PDLC systems are limited in application due to only being able to switch between a scattered white state and a transparent state and at times have a high voltage, and thus both systems still have problems to be overcome.
Light-controlling materials utilizing the guest-host system allow regulation of bright light and are thus attracting attention as a system suited for light-controlling applications. However, since the materials proposed so far (see, for example, JP-A No. 2000-347,224) required high voltage for addressing and do not have a memory property, they consume a large amount of power and still do not have a light-controlling property of a satisfactory level in some cases, and thus further improvement is demanded.